1. Field of the Invention
This invention relates to a method of measuring the basic amount contained in a basic aluminum sulfate solution which is employed in the removal of SO.sub.2 gas.
2. Description of the Prior Art
A solution of basic aluminum sulfate is produced when an aluminium sulfate solution is neutralized with a neutralizing agent, such as, calcium carbonate. Basic aluminum sulfate may be represented by the following chemical formula (1). EQU Al.sub.2 (SO.sub.4).sub.3.nAl.sub.2 O.sub.3 ( 1)
The amount of aluminum expressed as Al.sub.2 O.sub.3 in the above formula is termed the "basic amount," and is generally expressed in g/l. It is also widely known that the ratio of the basic amount with respect to the total amount of aluminum in a solution of basic aluminum sulfate is termed the "basicity." This basicity is calculated by the following expression (2). ##EQU1##
In order to calculate the basic amount in a solution of basic aluminum sulfate, pH measurements, electrical conductivity, and the like have been utilized. In this instance, there is a ternary relation between the calculated amount, the aluminum concentration, and the basic amount. Therefore, with these conventional methods, it is essential to thoroughly understand the ternary relationships in the solution. In addition, since the conventional methods are greatly influenced by impurities in the solution, accurate measurement of basic amount of a solution and its consequent basicity cannot easily be made.
The drawbacks of the above-mentioned conventional methods are visibly explained in FIGS. 2 to 4. FIG. 2 in which the relation between pH measurements and basic amounts is diagrammatically illustrated, shows that in comparison to comparatively large differences of the basic amounts of solutions, the difference in the pH values thereof is extremely small, viz, the difference of 2 g/l of basic amount is represented by a difference of only 0.2 to 0.3 corresponding pH values, said latter difference being too small for accurate measurement. FIG. 3 shows that the calibration of the basic amount contained in basic aluminum sulfate solutions by means of pH measurements is adversely affected by impurities contained in the solution, and one has to compensate for such impurities. In FIG. 4, which diagrammatically shows the relations between the basic amounts and the electrical conductivities of a pure solution in the range of .+-.2 g/l of basic amount. At about 4 g/l, the difference in conductivity is only .+-.13%, and said difference is as small as .+-.4.3% in case of a solution containing 2 g/l of Mg. This means that a calibration method by means of the measurement of electrical conductivity of a solution is not reliable.
The ability to easily and precisely to control basicity is particularly important when considering continuous processes. Thus, not every method allows for the continual measurement of the basicity without resulting in disruptions in the process, decreases in the efficiency and/or excessive variation in the process conditions and, in turn, the products or results produced.
Of special importance, in this respect, in the process wherein SO.sub.2 is removed from waste gases by absorption in a solution of basic aluminum sulfate. Such a process is carried out continuously and unless the basicity of the solutions generated and used during the process can be precisely controlled within specified limits, the process cannot be successfully carried out efficiently.